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Sear No. 3 Mine Sediment Pond Operations • The sediment pond at the Bear No. 3 Mine ie shown on Map 11-3. The pond is designed to contain the run-off Prom the l0 year 24 hour event. Dewatering of the pond will be required at any time the water level in the pond is above elevation 6003.5, in order to maintain the required 0.75 AF storage volume. Devaterinq of the water from a significant storm event will occur after the water has been allowed to settle for 24 to 48 hours. The outlet pipe for the ponfl consists oP a 12" CMP. The pipe vas installed incorrectly with the outlet being higher than the inlet. The invert of the inlet end of the pike is at elevation 6002.9. The invert of the outlet end of the pipe is at elevation 6003.2. For this reason the maximum sediment level in the pond is elevation 6002.9 and the minimum water level in the pond is elevation 6003.2. Therefore there is dead water storage in the pond between elevations 6002.9 and 6003.2. This dead water storage causes the "gap" in the stage filling diagram presented on drawing 11-3. Pond Devatering The pond will be dewatered by opening the normally closed slide gate on the 12" CMP pipe. Use the following formula for discharge under falling head to determine the time it texas to dewater the pond. DISCHARGE UNDER FALLING HEAD Assume area of pond must be constant with varinq • height. Use average surface area of pond. Surface Area of Pond. 13390 SQUARE FEET Diameter of Pipe 12 INCHES Area oP Pipe = a 0.7850 SQUARE FEET C = Loss Coefficient 0.65 H1 Beginning Head 2.7 FEET Elevation 6005.9 H2 = Ending Head 1.0 Elevation 6004.2 t = time to lower the surface level of the pond (H1-H2) t = 2A*(H1^.5 - H2^.5)/C*a*(2G) t 4206 t = 70.1 t = 1.2 t = 0.05 gpm = 2429 Cfs 5.4 ^.5 Seconds Minutes Hours . DAYS Gallons per Minute Cubic Feat par Second Reference Civil Engineering Handbook, Urquhart, Fourth Edition, Page 4-34. 11-12 Nev August 4, 1993 •